1. Field of the Invention
The present invention relates to valves typically used on downhole tools in oil and gas wells and in particular, though not exclusively, to a zonal control valve to regulate production flow rate from several hydrocarbon bearing zones within a well.
2. Description of Related Art
In the drilling of oil and gas wells, well bores are now typically drilled in a deviated path so that the well intersects as many hydrocarbon bearing zones over the greatest distances possible. By perforating the well casing or tubing at each zone or across a zone, the well can be produced at a number of locations simultaneously and thereby maximise production. However, this simultaneous production requires to be carefully controlled as production will flow from one zone to another zone rather than to surface, if the pressure between each zone varies.
Prior art flow control systems have been based on inserting production tubing into the well with packers between each of the producing zones. At each zone, a valve is located in the tubing, to allow production to flow from the zone into the tubing and up to surface. Typically only a single zone is produced at a time to prevent cross-talk. If more than one zone is produced at a time, the pressures between the zones must be balanced. This is achieved by locating a valve at each zone with each valve has a selected size of orifice or opening so that the flow rate through each valve is equalised into the tubing. A control line, typically hydraulic, is run to each valve and the valves are then opened as required.
There are a number of disadvantages of such control systems. In order to use a number of valves, the flow rate from each zone must be determined. This is not a simple task as flow rates from each zone can vary. The valves are then selected on the basis of the required flow rate in the tubing and these valves must be correctly positioned on the tubing in the well. Once positioned, these valves are either ‘on’ or ‘off’, so that no adjustment of the flow rate through the valve can be made when the valve is downhole. Any changes in the flow rate from a zone result in either the respective valve having to be closed which losses production from the zone, or the tubing must be removed so that the valve can be replaced and the tubing run back in the well. This causes downtime in production. Additionally a control line is required for each valve, with each control line having to be run to surface, operated and monitored individually.
U.S. Pat. No. 6,782,952 proposes a stepping valve which allows the valve to be incrementally opened or closed. This provides selective throttling of the flow downhole.
While this valve has this advantage, it also has a number of disadvantages. The valve is based on a hydraulically actuated sliding sleeve. An inherent problem in sliding sleeve valves is that these valves are long devices. This is because there must be sufficient length in the tool as the sleeve must be able to move longitudinally across a stroke distance between the open and closed positions of the valve. Additionally, this valve requires two control lines to operate. Thus in a well having a number of zones, valuable space is required to run two control lines to each valve at each zone.